A DC-to-DC converter drives a DC load such as one or more light-emitting diodes (LEDs) or the like. The converter includes a DC-to-AC inverter in a primary circuit. The inverter generates a switched AC voltage, which is applied to the primary winding of an isolation transformer. A secondary winding provides an input to an AC-to-DC rectifier in a secondary circuit. The rectifier produces a DC voltage, which is applied to the load. The magnitude of a load current flowing through the load is sensed and compared to a reference magnitude. An operational amplifier or other comparator in the secondary circuit generates a feedback signal in response to a difference between the sensed current magnitude and the reference magnitude. The feedback signal is applied to an optocoupler having an input stage in the secondary circuit and having an output stage in the primary circuit. The output stage of the optocoupler has a variable impedance that is incorporated into a voltage superposition circuit. The voltage superposition circuit generates a superposition voltage that is applied to an input node of a current control circuit. The current control circuit controls a frequency control current flowing from a control terminal of a switch controller integrated circuit in the DC-to-AC inverter. The superposition voltage modifies the frequency control current. The switch controller is responsive to the frequency control current to vary the frequency of the switched AC voltage. Varying the frequency of the AC voltage varies the magnitude of the load current. In some circumstances, the output stage of the optocoupler may be shorted such that the superposition voltage is maximized. The maximized superposition voltage causes a minimum control current, which results in a minimum frequency of the AC voltage. The minimum frequency of the AC voltage may cause the load current through the load to be unacceptably great.